. A central goal of neurobiology is to identify the cellular mechanisms that enable animals to memorize novel stimuli and learn new behaviors. This project seeks to forge a link between the cellular and the behavioral aspects of learning by studying the synaptic properties of neurons within the avian brain that mediate the acquisition and production of learned songs. The goal of this study is to determine whether certain forms of long-lasting synaptic modification that can be observed in vitro provide the cellular mechanism for avian song learning. To further understand whether synaptic modification within the song system underlies avian vocal learning, experiments will test whether song system synapses can be strengthened through certain patterns of activation, whether this capacity is developmentally restricted, and whether acoustical isolation and castration, both of which lengthen the period of song learning, also lengthen the period in which synaptic modification can occur. This project will also examine the effects of acoustical isolation on the kinetics of N-methyl-D-aspartate(NMDA) receptor-mediated excitatory postsynaptic currents (EPSCs) within three forebrain song control nuclei, since NMDA receptors mediate several forms of synaptic plasticity within the vertebrate central nervous system, and because maturational changes in the kinetics of NMDA receptors are delayed in some developmentally plastic systems by sensory deprivation. Whole-cell voltage clamp recordings will be made in an in vitro brain slice to characterize the properties of NMDA-EPSCs within the song control circuit during development in both normal birds and in acoustical isolates. As a prelude to these experiments, the role of glutamatergic transmission in several forebrain song control nuclei will be described. A related set of experiments will examine the effects of steroid hormones on synaptic properties of the song system. Steroid hormones play a key role in ending vocal plasticity.Androgens "crystallize" song, transforming the structurally variable juvenile song to the acoustically stereotyped song of the adult. The final set of experiments will examine the effects of castration and androgen treatment on synaptic transmission within the song system, especially on NMDA receptor mediated transmission, and on the tonic level of inhibition, again using an in vitro brain slice preparation.